This invention is directed to a current comparator capacitance bridge for the measurement of inductance, and, in particular, to a frequency fluctuation compensation for such a bridge.
A current-comparator capacitance bridge which is adapted to the measurement of an inductance, such as an inductive shunt reactor, by providing the means to reverse the phase of the reactor current and tangent .delta. balance is described in the publication, "Measurement of Shunt Reactor Loss of High Voltage with an Alternating Current-Comparator Bridge", W. J. M. Moore et al, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-92, No. 2, pp 662-667, March/April 1973. The comparison of an inductance to a capacitive reference results in a bridge which is frequency sensitive.
To overcome this deficiency, a modified current-comparator capacitance bridge was proposed in the publication, "Mesures des Pertes dans les Reactances Shunt EHT par un pont C-tan .delta. Insensibilise aux Fluctuations de la Frequence du Reseau", R. Malewski et al, Conference Digest IEEE Canadian Communications and Power Conference, Montreal, Canada, Oct. 18-20, 1978, ch. 1373-0, Reg. 7, pp 487-491. This proposed modified bridge was used in a study entitled, "Interlaboratory Comparison on EHV Shunt Reactor Loss Measurements", R. Malewski et al, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-99, No. 4, pp 1634-1641, July/Aug. 1980. In this modified bridge, the comparator winding in the reference capacitor arm of the bridge, is driven by a current proportional to the integral of a voltage which is a reduced replica of the applied high voltage to the bridge. The frequency characteristics of the reference arm of this modified bridge thus simulate those of the reactor, and the operating difficulties of the current-comparator-based capacitance bridge due to slight variations in frequency are overcome. The accuracy and long-term stability of the current-comparator have been exchanged, however, for those of the electronic circuitry that has been inserted between the reference capacitor and the comparator winding.
A second method and apparatus for rendering a high voltage, current-comparator capacitance bridge insensitive to frequency fluctuations when used for measuring inductance is described in U.S. Pat. No. 4,477,772 issued on Oct. 16, 1984, in the name of Eddy So. However, this method and the first method referred to above, both require gain and phase adjustments.